This invention relates to the single stage AC etching of aluminum capacitor foil to produce a foil with a metallic core, and hence good mechanical properties, as well as improved capacitance. The electrolyte system contains hydrochloric and oxalic acids and aluminum ions. The etching is carried out at 30.degree. to 45.degree. C. and 16.4 to 36 Hz frequency.
AC etching has been used to produce aluminum articles such as lithographic plates and capacitor foil. It also has been used to electropolish aluminum articles prior to metal plating.
The prior art has discussed the difficulties in obtaining an even or uniform etch structure and has overcome these difficulties in a variety of ways, e.g., interrupting the etch process to apply protective coatings, carrying out the etch process in stages and using additives in the electrolyte bath to control pit size or to increase mechanical strength of the foil.
Another problem has been to prevent the precipitation of aluminum hydroxide, formed during etching, on or into the etched surfaces.
The resolution of these problems has led to prior art processes in which the etch conditions are carefully controlled to provide the desired increase in surface area and, particularly for capacitor foil, with little change in mechanical strength. Such a process is described by M. Arora and J. J. Randall, Jr., in copending application Ser. No. 187,241, filed Sept. 15, 1980 now U.S. Pat. No. 4,279,714, that utilizes a phosphate in the hydrochloric etch bath. Another such process is described by M. Arora, E. J. Paquette and A. B. McPherson in copending application Ser. No. 210,940, filed Nov. 28, 1980, that utilizes both phosphoric and nitric acids in the hydrochloric acid etch bath. Another process is described by M. Arora in copending application Ser. No. 188,637, filed Sept. 19, 1980, that utilizes an intermittent etch scheme.
Still other processes utilize both DC and AC etching to form tunnels in one stage and then broaden them in the next stage without impairing the mechanical strength of the foil.